classification of element

• Mendeleev's Periodic Law states that the properties of elements are a periodic function of their atomic masses
• Moseley showed that atomic number is a more fundamental property of an element than its atomic mass
• The Modern Periodic Law states that the physical and chemical properties of elements are periodic functions of their atomic numbers
• Types of elements include s-, p-, d-, and f-blocks
• Main group elements or representative elements are the s- and p-block elements
• 1. block elements include Group-1 (Alkali metals) and Group-2 elements (Alkaline earth metals) with ns1 and ns2 outermost electronic configurations respectively
• 1. block elements belong to groups 13 to 18 with outermost electronic configuration ns2 np1-6
• 1. block elements (Transition elements) have outer electronic configuration (n-1) d1-10 ns1-2
• 1. block elements (Inner-transition Series) include Lanthanoids and Actinoids with characteristic outer electronic configurations (n-2) f1-14 (n-1) d0-1 ns2
• Noble gases in group 18 have outermost electronic configuration ns2 np6, except for Helium which has 1s2 configuration
• Atomic properties include atomic radius, ionic radius, ionisation energy, electronegativity, electron affinity, and valence
• Atomic radius is the distance from the nucleus to the outermost shell of electrons
• In a period, atomic radius decreases from left to right, while in a group, it increases from top to bottom
• Covalent radius is half the inter-nuclear distance between two similar atoms covalently bonded by a single covalent bond
• Van der Waals' radius is half the inter-nuclear separation between two similar adjacent atoms in the solid state
• Metallic radius is half the distance between the nuclei of two adjacent metal atoms in a close-packed lattice
• Ionic radius is the effective distance from the nucleus of an ion up to which it influences its electron cloud
• Cations are smaller, but anions are larger than the parent atom
• Ionisation enthalpy is the enthalpy change when an electron is removed from a gaseous phase atom or ion in its ground state
• Ionisation enthalpy generally increases from left to right in a period and decreases from top to bottom in a group
• Electron gain enthalpy is the enthalpy change when an isolated gaseous atom or ion adds an electron to form the corresponding anion
• Electron gain enthalpy increases from left to right in a period and decreases from top to bottom in a group
• Electronegativity is the relative tendency of an atom in a molecule to attract shared electrons towards itself
• Electronegativity increases from left to right in a period and decreases from top to bottom in a group
• Valence electrons are the electrons present in the outermost shell of an atom
• Valency of an element is the number of atoms it combines with to form compounds
• Valency first increases then decreases from left to right in a period, while it remains constant from top to bottom in a group
• Electropositive or metallic character is the tendency of an element to lose electrons and form positive ions (cations)
• Electropositive or metallic character decreases from left to right in a period and increases from top to bottom in a group
• Electronegative or non-metallic character is the tendency of an element to accept electrons and form anions
• Electro-negative or non-metallic characters increase from left to right in a period and decrease from top to bottom in a group

  
• Reactivity of metals decreases from left to right in a period and increases from top to bottom in a group

  
• Solubility of alkali metal carbonates and bicarbonates in water increases down the group (From Lithium to Caesium)
• Basic strength of alkaline earth metal hydroxide in water increases down the group (From Beryllium to Barium)
• Thermal stability of carbonates of alkali and alkaline earth metals:

  • Except lithium carbonate (LiCO3), the carbonates of all other alkali metals are stable towards heat
  • LiCO3 decomposes on heating to give lithium oxide (LiCO3)
  • The carbonates of alkaline earth metals decompose to give corresponding oxide and CO2 gas
  • The decomposition temperature for alkaline earth metal carbonates increases as we go down the group
• Anomalous properties of second period elements are attributed to their small size, large charge/radius ratio, high electronegativity, and non-availability of d-orbitals in their valence shell
• The first ionisation enthalpy of magnesium is much smaller than that of sodium because Mg+ formed after losing the first electron still has one more electron in its outermost (3s) orbital
• Major differences between metals and non-metals:

  • Metals are electropositive, while non-metals are electronegative
  • Types of ions formed: Cations (positively charged) for metals, Anions (negatively charged) for non-metals
  • Reaction with acids: Active metals displace hydrogen, while non-metals do not displace hydrogen
  • Oxides: Metals form basic oxides, while non-metals form acidic oxides
• In the second period from Li to Ne:

  • Element with the highest first ionisation energy: Ne
  • Element with the highest electronegativity: F
  • Element with the largest atomic radius: Li
• Arrangement of N2, O2, F2, Cl2:

  • Increasing order of bond dissociation energy: F2 < Cl2 < O2 < N2